The endogenous protein content of ruminant proximal duodenal digesta
Protein arriving at the ruminant proximal duodenum consists of microbial protein, undegraded feed protein and endogenous protein. In this study, endogenous protein is defined as that fraction of the digesta derived from the animal itself (e.g. enzymes, plasma proteins, sloughed cells and mucus), not including any endogenous protein which may have been incorporated into the microorganisms. Recent feeding schemes (e.g. ARC 1980, 1984) require an accurate value of the degradability of feed in the rumen. When degradability is determined in vivo failure to account for a quantity of endogenous protein in the proximal duodenal digesta results in an underestimate of the degradability of a ration. Direct estimates of the endogenous protein content of proximal duodenal digesta are therefore required. This thesis describes the development of a method to do this and its application to sheep and cattle. The approach adopted, based on the concept of isotope dilution, involved a continuous intravenous infusion of L-[4,5-3H]-leucine. This resulted in all body protein becoming labelled. Any label detected in the duodenal digesta must therefore be derived from the animal itself. A comparison of the specific activity of duodenal digesta with that of suitable precursor proteins provided an estimate of the proportion of duodenal digesta of endogenous origin. Interestingly, the bacterial fraction of duodenal digesta was also labelled. This indicated that bacteria were utilising an endogenous source of leucine and circumstantial evidence suggested that this was derived largely from the rumen epithelium. The validity of the L-[4,5-3H]-leucine technique was investigated using three sheep fed an essentially protein free diet. Values of endogenous protein flow derived via the proposed technique (2.6 + 1.00 g N/day) were compared with those calculated by difference (2.1 + 0.92 g N/day). A possible dietary influence on the endogenous protein content of ruminant proximal duodenal digesta was examined. An estimate of 2.6 + 0.58 g N/day was derived for three sheep fed a concentrate diet and 2.1 + 0.22 g N/day for three sheep fed long hay. Thus, contrary to previous suggestions, no significant dietary effect was observed. Possible reasons for this are discussed. The proposed technique was also applied to derive estimates in cattle. A value of 11.3 + 1.73 g N/day was obtained for four steers (170 kg) fed silage supplemented with fishmeal. The continuous intravenous infusion of L-[4,5-3H]-leucine method is the first technique to provide a direct determination of the endogenous protein component of ruminant proximal duodenal digesta which can be applied to any dietary regime and used in sheep and cattle.